Many compounds are already known as 1,4-dihydropyridine derivatives. The majority of these known 1,4-dihydropyridine derivatives have a pharmacological activity with respect to the circulatory system. For other pharmacological activities, there have been just a few reports for those having an anti-inflammatory action, liver protecting action, etc.
On the other hand, "acquired resistance ", where the effect of the anti-cancer drug is lost during the treatment, is becoming a problem in the chemotherapy of cancers. Multidrug resistance which exhibited against a variety of anti-cancer drugs is becoming an important problem. As a method for overcoming this multidrug resistance, it has been reported that combined administration of an anti-cancer drug and some calcium antagonists (1,4-dihydropyridine compounds such as nicardipine) is effective (Cancer Res., 41, 1967-1972 (1981), Cancer and Chemotherapy, vol. 15, 2848 (1988)).
Further, Japanese Unexamined Patent Publication (Kokai) No. 2-40383, Japanese Unexamined Patent Publication (Kokai) No. 2-240081, describe a 1,4-dihydropyridine compound having a dioxene ring or a dithiene ring at its 4-position, Japanese Unexamined Patent Publication (Kokai) No. 5-117235 and Japanese Unexamined Patent Publication (Kokai) No. 2-138221 describes a 1,4-dihydropyridine compound having an aromatic group such as a phenyl group bonded at its 4-position, and WO96/04268, WO97/28125, WO97/28152 describe a 1,4-dihydropyridine compound having, an alkyl group etc., bonded at its 4-position, as those having an action for overcoming resistance to an anti-cancer drug.
However, the inventions described in the above references (i.e., Cancer Res., 41, 1967-1972 (1981) and Cancer and Chemotherapy, vol. 15, 2848 (1988)) use calcium antagonists for overcoming resistance to anti-cancer drugs and have the defect that they are not necessarily practical in terms of side effects. That is, calcium channel blockers inherently have a powerful action and act on the heart, blood vessels, etc. even in very small amounts, and therefore, if large amounts of these drugs are used, there is the problem that inconvenient effects are to be caused on the heart and circulatory system.
Further, among the 1,4-dihydropyridines described in the above references, Japanese Unexamined Patent Publication (Kokai) No. 2-40383, Japanese Unexamined Patent Publication (Kokai) No. 2-240081, WO96/04268, WO97/28125, WO97/28152 etc., there are preferable compounds having an action to increase the effect of anti-cancer drugs or an action for overcoming the resistance to an anti-cancer drug and further have almost no calcium channel blocking action. However, the chemical structures of the 1,4-dihydropyridine compounds described in these publications are clearly different from those of the present invention.